1. Field of the Invention
The present invention relates to a novel thiol compound useful as a chain transfer agent, a copolymer in radical copolymerization using the thiol compound as the chain transfer agent, and methods for producing the same. More particularly, the present invention relates to the copolymer suitably used for coating film in lithography such as resist film and anti-reflective coating, and the method for producing the same, as well as the novel thiol compound useful as the chain transfer agent in the production of the copolymer.
2. Description of Related Art
In semiconductor lithography, the formation of finer patterns has been required in conjunction with increase of integration degree. It is essential for micropatterning techniques to make a wavelength of irradiation light source shorter. Currently, the lithography by krypton fluoride (KrF) excimer laser (wavelength 248 nm) has become a mainstream, and the micropatterning techniques of 100 nm linewidth or less by argon fluoride (ArF) excimer laser (wavelength 193 nm) is coming into practical use. The micropatterning techniques using fluorine dimer (F2) excimer laser (wavelength 157 nm), extreme-ultraviolet ray (EUV), X-ray, electron beam, and the like are in developmental stages.
The resist polymer used for these lithography technologies has essentially a repeating unit having a non-polar substitute which is decomposed by an acid and becomes soluble in an alkali developer and a repeating unit having a polar group to enhance adhesion to a substrate. And if necessary, the resist polymer is comprising a repeating unit having non-polar substitutes to regulate solubility in a resist solvent or the alkali developer. As these repeating units, for example, in the KrF lithography, hydroxystyrenes and derivatives thereof have been primarily used, and in the ArF lithography, (meth)acrylates and derivatives thereof and the like have been considered because the hydroxystyrenes absorb the light with a wavelength of 193 nm.
As specific examples of such resist polymers, copolymers of (meth)acrylic monomer with styrene type monomer (see e.g., Patent References 1 to 4), polymers containing hydroxystyrene which is partially protected with acetal (see e.g., Patent References 5 to 8) and the like are known in the KrF lithography, and copolymers of (meth)acrylic monomer having lactone structure (see e.g., Patent references 9 to 10) are known in the ArF lithography.
Whereas, even in the polymer comprising the repeating unit having the polar group to enhance the adhesion to the substrate as the above, it is a problem that finer patterns cannot be achieved because end-groups of the polymer derived from a initiator or a chain transfer agent used at the polymerization don't have sufficient adhesion property.
Also, in the resist pattern formation, it is a problem that resist patterns collapse due to surface tension of the washing water vaporizing in a drying process after development and rinsing. Especially, the pattern collapse occurs easily because a pattern-substrate contacting area becomes small when the pattern becomes finer. To avoid this, it is necessary to keep an aspect ratio (height/width) low, but on the other hand, it is required to make a coating film thick in order to satisfy dry etching resistance of the pattern, that is, to make the aspect ratio of the resist pattern high. Thus, the resist polymer with higher adhesion to the substrate where the aspect ratio can be increased and the pattern collapse does not occur even in fine pattern formation has been needed.
Thus, the several methods using a polymerization initiator and a chain transfer agent having a polar group have been studied to enhance adhesion of the resist polymer to the substrate. As an example using the polymerization initiator having a polar group, the method using the polymerization initiator having an oxygen-containing group or a substituted or unsubstituted amino group in the molecule is known (see e.g., Patent Reference 11). In the reference, hydroxyl group, carboxyl group, substituted oxyl group, substituted oxycarbonyl group, acyl group, substituted or unsubstituted carbamoyl group, hydroxyimino group, and substituted or unsubstituted oxyimino group are disclosed as the oxygen-containing groups. However, in the method of using carboxyl group, it has been a problem that swelling occurs easily in the alkali developing process after the exposure because of the strong hydrophilic property of carboxyl group, and then no stable pattern is obtained in finer lithography even if the alkali-solubility and the substrate adhesion can be improved. In the method of using hydroxyl group, substituted oxyl group, substituted oxycarbonyl group, acyl group and the like, it is insufficient to improve the adhesion to the substrate, because the polarity of these groups is weak. Furthermore, the method of using the oxygen-containing group comprising nitrogen atoms such as substituted or unsubstituted carbamoyl group, hydroxyimino group or substituted or un-substituted oxyimino group or the like, and the method of using the substituted or unsubstituted amino group are not practical, because these nitrogen-containing groups trap the acid generated from acid generator and make sensitivity lower.
At the same time, as examples using the chain transfer agent having the polar group, the method of using carboxyl group-containing thiol such as mercaptoacetic acid and mercaptopropionic acid as the chain transfer agent (see e.g., Patent Reference 12) and the method of using ester compounds thereof or hydroxyl group-containing thiol such as mercaptoethanol are known (see e.g., Patent Reference 13 to 14). However, also in these methods, as with the case of the above polymerization initiator, it is a problem that the swelling easily occurs at the alkali development in the method of using carboxyl group-containing thiol, and the adhesion to the substrate is insufficient in the method of using the ester compound or hydroxyl group-containing thiol, and both cannot reach to practical levels.
Moreover, in the lithography on a high reflective substrate, it is difficult to trace fine resist patterns due to halation of reflected lights. To solve this issue, an anti-reflective coating polymer is coated under a resist coating film and absorbs the reflected lights of the substrate. Furthermore, in the multi-layer resist application for progressed lithography technology, a bottom layer coating polymer is coated under the thin resist coating film and imaged by dry etching. For all these coating polymers, the adhesion to the substrate is important, and more excellent adhesion is required.                Patent Reference 1                    JP-A-59-45439                        Patent Reference 2                    JP-A-5-113667                        Patent Reference 3                    JP-A-7-209868                        Patent Reference 4                    JP-A-11-65120                        Patent Reference 5                    JP-A-62-115440                        Patent Reference 6                    JP-A-4-219757                        Patent Reference 7                    JP-A-3-223860                        Patent Reference 8                    JP-A-4-104251                        Patent Reference 9                    JP-A-9-73173                        Patent Reference 10                    JP-A-10-239846                        Patent Reference 11                    JP-A-2002-20424                        Patent Reference 12                    JP-A-10-55069                        Patent Reference 13                    JP-A-2000-19737                        Patent Reference 14                    JP-A-2001-117231                        